Neutron Detection

ABSTRACT

A neutron detector includes a microchannel plate having a structure that defines a plurality of microchannels, and layers of materials disposed on walls of the microchannels. The layers include a layer of neutron sensitive material, a layer of semiconducting material, and a layer of electron emissive material. For example, the layer of neutron sensitive material can include at least one of hafnium (Hf), samarium (Sm), erbium (Er), neodymium (Nd), tantalum (Ta), lutetium (Lu), europium (Eu), dysposium (Dy), or thulium (Tm).

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application61/894,867, filed on Oct. 23, 2013. This application is related to U.S.application Ser. No. 13/069,898, filed on Mar. 23, 2011, issued as U.S.Pat. No. 8,507,872 on Aug. 13, 2013. The above applications areincorporated by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under DOE contractDE-SC0009657. The government has certain rights in the invention.

BACKGROUND

This invention relates to neutron detection.

Neutron-sensitive microchannel plates (MCPs) can be used to detectspecial nuclear materials (SNM), such as plutonium, or can be used inneutron imaging. A microchannel plate can be formed by bonding a glassplate between an input electrode and an output electrode, and providinga high voltage direct current (DC) field between the electrodes. Theglass plate is perforated with a substantially regular, parallel arrayof microscopic channels, for example, cylindrical and hollow channels.Each channel, which can serve as an independent electron multiplier, hasan inner wall surface formed of a semi-conductive and electron emissivelayer.

The glass plate can be doped with, for example, boron-10, which cancapture neutrons in reactions that generate lithium-7 and alphaparticles. As the lithium-7 and alpha particles enter nearby channelsand collide against the wall surfaces to produce secondary electrons, acascade of electrons can be formed as the secondary electrons acceleratealong the channels (due to the DC field), and collide against the wallsurfaces farther along the channels, thereby increasing the number ofsecondary electrons. The electron cascades develop along the channelsand are amplified into detectable signals that are electronicallyregistered and processed to construct a digital image. The resultantintensity map or image corresponds to the variation in neutron fluxstriking the microchannel plate's surface. Contrast differences withinthe image of a sample can be used to infer physical and chemicalproperties.

SUMMARY

In general, in one aspect, an apparatus comprises a microchannel platecomprising a structure that defines a plurality of microchannels; andlayers of materials disposed on walls of the microchannels, the layersincluding a layer of neutron sensitive material, a layer ofsemiconducting material, and a layer of electron emissive material. Thelayer of neutron sensitive material includes at least one of hafnium(Hf), samarium (Sm), erbium (Er), neodymium (Nd), tantalum (Ta),lutetium (Lu), europium (Eu), dysposium (Dy), or thulium (Tm).

Implementations of the apparatus may include one or more of thefollowing features. As compared with using a layer of neutron sensitivematerial that includes compounds containing boron-10, lithium-6, andgadolinium, using materials from this group (hafnium, samarium, erbium,neodymium, tantalum, lutetium, europium, dysposium, and/or thulium) mayfurther enhance the microchannel plate sensitivity to higher energyneutrons, for example, those with energies in the “epithermal” energyrange (e.g., 0.1 eV up to 1000 eV). The layer of neutron sensitivematerial can include at least 50 mol % of neutron sensitive material.The layer of neutron sensitive material can include at least one ofboron-10, lithium-6, or gadolinium. The layer of neutron sensitivematerial can include a compound having at least one of boron-10 orgadolinium. The compound can include at least one of boron-10 oxide,boron-10 nitride, lithium-6 oxide, or gadolinium oxide. The layer ofneutron sensitive material can have a thickness in a range from 0.1 to 5microns. The layer of semiconducting material can have a thickness in arange from 25 to 1000 nm. The layer of electron emissive material canhave a thickness in a range from 3 to 20 nm. The structure can includeglass. The glass can have less than 25 mol % lead (Pb). In someexamples, the glass can have less than 1 mol % lead (Pb). In someexamples, the glass can have more than 1 mol % lead (Pb). The glass canhave less than 25 mol % of elements having an atomic number greater than34. In some examples, the glass can have less than 1 mol % of elementshaving an atomic number greater than 34. In some examples, the glass canhave more than 1 mol % of elements having an atomic number greater than34. The glass can have less than 20 mol % of neutron sensitive material.In some examples, the glass can have more than 1 mol % of neutronsensitive material. In some examples, the glass can have less than 1 mol% of neutron sensitive material. The layer of neutron sensitive materialcan be without glass. The semiconducting material can includeAlZn_(x)O_(y) alloy (x and y being positive integers). The electronemissive material can include at least one of aluminum oxide (Al₂O₃) ormagnesium oxide (MgO). Each of at least some of the microchannels canhave a diameter in a range between 5 to 10 microns. Each of at leastsome of the microchannels can have a diameter less than 150 microns.Each of at least some of the microchannels can have a length that is atleast 10 times the diameter of the microchannel. Each of at least someof the microchannels can have a circular, a square, a rectangular, or ahexagonal cross section. The apparatus can include an image sensor todetect the secondary electron emissions. The apparatus can include agamma ray detector to detect gamma rays, and a coincidence unit todetermine whether a first signal output from the gamma ray detectorindicating detection of a gamma ray occurs within a predetermined timeperiod after a second signal output from the microchannel plateindicating detection of at least one of a neutron or a gamma ray. Theneutron sensitive material can include at least one of boron-10 orgadolinium. The microchannel plate can include an input electrode, anoutput electrode, and a glass plate comprising the microchannels, andthe apparatus can further include a data processor to determine whethera neutron has been detected based on first information derived from afirst charge induced on the input electrode and/or second informationderived from a second charge induced on the output electrode. The dataprocessor can be configured to calculate a ratio between the firstcharge and the second charge, calculate a sum of the first and secondcharges, and determine whether a neutron has been detected based on theratio and the sum. The data processor can be configured to calculate aparameter value based on dividing the ratio by the sum, compare theparameter value with a predetermined range of values, and determine thata neutron has been detected when the parameter value is within thepredetermined range of values. The neutron sensitive material caninclude at least one of boron-10 or lithium-6.

In general, in another aspect, a method of fabricating a microchannelplate includes fabricating a structure that defines a plurality ofmicrochannels; and depositing a layer of neutron sensitive material, alayer of semiconducting material, and a layer of electron emissivematerial on walls of the microchannels. The layer of neutron sensitivematerial comprises at least one of hafnium (Hf), samarium (Sm), erbium(Er), neodymium (Nd), tantalum (Ta), lutetium (Lu), europium (Eu),dysposium (Dy), or thulium (Tm).

Implementations of the method may include one or more of the followingfeatures. As compared with using a layer of neutron sensitive materialthat includes compounds containing boron-10, lithium-6, and gadolinium,using materials from this group (hafnium, samarium, erbium, neodymium,tantalum, lutetium, europium, dysposium, and/or thulium) may furtherenhance the microchannel plate sensitivity to higher energy neutrons,for example, those with energies in the “epithermal” energy range (e.g.,0.1 eV up to 1000 eV). Atomic layer deposition can be used to deposit alayer of neutron sensitive material. The layer of neutron sensitivematerial can include at least one of boron-10 or gadolinium. The layerof neutron sensitive material can include a compound material having atleast one of boron-10, gadolinium, or lithium-6. The layer of neutronsensitive material can include at least one of boron-10 oxide, boron-10nitride, lithium-6 oxide, or gadolinium oxide. The layer of neutronsensitive material can have a thickness in a range from 0.1 to 5microns. The layer of neutron sensitive material can include at leasttwo of boron-10, lithium-6, or gadolinium. Atomic layer deposition canbe used to deposit the layer of semiconducting material. The layer ofsemiconducting material can have a thickness in a range from 20 to 1000nm. Atomic layer deposition can be used to deposit the layer of electronemissive material. The layer of electron emissive material can have athickness in a range from 3 to 20 nm. The structure that defines aplurality of microchannels can be fabricated using a plurality of fiberseach including a soluble core and a layer of cladding surrounding thesoluble core, and removing the soluble core to form microchannels. Thestructure can be fabricated using glass. The structure can be fabricatedusing glass that has less than 25 mol % lead (Pb). In some examples, thestructure can be fabricated using glass that has less than 1 mol % lead(Pb). In some examples, the structure can be fabricated using glass thathas more than 1 mol % lead (Pb). The structure can be fabricated usingglass that has less than 25 mol % of elements having an atomic numbergreater than 34. In some examples, the structure can be fabricated usingglass that has less than 1 mol % of elements having an atomic numbergreater than 34. In some examples, the structure can be fabricated usingglass that has more than 1 mol % of elements having an atomic numbergreater than 34. The method can include forming an input electrode on aninput surface of the microchannel plate and an output electrode on anoutput surface of the microchannel plate. The layer of neutron sensitivematerial can be deposited on the walls of the microchannels first,followed by depositing the layer of semiconducting material, andfollowed by depositing the layer of electron emissive material.

In general, in another aspect, a method of detecting neutrons includesusing a layer of neutron sensitive material formed on a wall of amicrochannel of a microchannel plate to capture a neutron and generateat least one reactant particle, the microchannel plate including a glassplate having a structure that defines the microchannel, the glass plateincluding glass having less than 25 mol % lead (Pb) and less than 20 mol% of the neutron sensitive material; detecting secondary electrons thatare generated based on an interaction between the reactant particle anda layer of electron emissive material on the wall of the microchannel;and generating a first signal indicating detection of a neutron. Theneutron sensitive material comprises at least one of hafnium (Hf),samarium (Sm), erbium (Er), neodymium (Nd), tantalum (Ta), lutetium(Lu), europium (Eu), dysposium (Dy), or thulium (Tm).

Implementations of the method may include one or more of the followingfeatures. As compared with using a layer of neutron sensitive materialthat includes compounds containing boron-10, lithium-6, and gadolinium,using materials from this group (hafnium, samarium, erbium, neodymium,tantalum, lutetium, europium, dysposium, and/or thulium) may furtherenhance the microchannel plate sensitivity to higher energy neutrons,for example, those with energies in the “epithermal” energy range (e.g.,0.1 eV up to 1000 eV). The method can include generating a second signalindicating detection of the secondary electrons, generating a thirdsignal indicating detection of a gamma ray, determining whether thethird signal occurred within a specified time period after occurrence ofthe second signal, and generating the first signal only if the thirdsignal occurred within the specified time period after occurrence of thesecond signal. The glass plate can include glass having less than 1 mol% lead (Pb). The glass plate can include glass having less than 0.1 mol% of the neutron sensitive material.

In general, in another aspect, an apparatus includes a microsphere platehaving a plurality of microspheres that define interstices between themicrospheres; and layers of materials disposed on surfaces of themicrospheres, the layers including a layer of neutron sensitivematerial, a layer of semiconducting material, and a layer of electronemissive material. The layer of neutron sensitive material comprises atleast one of hafnium (Hf), samarium (Sm), erbium (Er), neodymium (Nd),tantalum (Ta), lutetium (Lu), europium (Eu), dysposium (Dy), or thulium(Tm).

Implementations of the apparatus may include the following feature. Ascompared with using a layer of neutron sensitive material that includescompounds containing boron-10, lithium-6, and gadolinium, usingmaterials from this group (hafnium, samarium, erbium, neodymium,tantalum, lutetium, europium, dysposium, and/or thulium) may furtherenhance the microsphere plate sensitivity to higher energy neutrons, forexample, those with energies in the “epithermal” energy range (e.g., 0.1eV up to 1000 eV).

In general, in another aspect, an apparatus includes a microfiber platehaving a plurality of microfibers that define interstices between themicrofibers; and layers of materials disposed on surfaces of themicrofibers, the layers including a layer of neutron sensitive material,a layer of semiconducting material, and a layer of electron emissivematerial. The layer of neutron sensitive material comprises at least oneof hafnium (Hf), samarium (Sm), erbium (Er), neodymium (Nd), tantalum(Ta), lutetium (Lu), europium (Eu), dysposium (Dy), or thulium (Tm).

Implementations of the apparatus may include the following feature. Ascompared with using a layer of neutron sensitive material that includescompounds containing boron-10, lithium-6, and gadolinium, usingmaterials from this group (hafnium, samarium, erbium, neodymium,tantalum, lutetium, europium, dysposium, and/or thulium) may furtherenhance the microfiber plate sensitivity to higher energy neutrons, forexample, those with energies in the “epithermal” energy range (e.g., 0.1eV up to 1000 eV).

DESCRIPTION OF DRAWINGS

FIG. 1A shows a cut-out view of an example microchannel plate.

FIG. 1B shows a cross-sectional view of the microchannel plate of FIG.1A along a plane perpendicular to the lengthwise direction ofmicrochannels.

FIG. 1C shows a cross-sectional view of the microchannel plate of FIG.1A along a plane parallel to the lengthwise direction of microchannels.

FIG. 2 shows an AB sequence for atomic layer deposition of a monolayerof a compound AB.

FIG. 3 is a diagram showing microchannels in a glass plate.

FIG. 4 is a diagram showing the development of an avalanche of secondaryelectrons.

FIGS. 5 and 6 are block diagrams of neutron detectors.

FIGS. 7 to 9 are flow diagrams of processes.

FIG. 10 is a block diagram of a neutron detector.

DETAILED DESCRIPTION

Referring to FIGS. 1A, 1B, and 1C, a neutron detector having a highsensitivity to neutrons and low sensitivity to gamma rays can befabricated by using a microchannel plate 100 having a glass plate 102made of low-Z glass without lead or other high mass components(materials having atomic numbers larger than 34). The glass plate 102has a substantially regular, parallel array of microscopic channels 110each having a diameter of about, for example, 5 to 10 microns (μm). Eachchannel can have, for example, a circular, square, rectangle, or hexagoncross sectional shape. FIG. 1A shows a cut-out view of the microchannelplate 100; FIG. 1B shows a cross-sectional view of the microchannelplate 100 along a plane perpendicular to the lengthwise direction of themicrochannels 110; and FIG. 1C shows a cross-sectional view of themicrochannel plate 100 along a plane parallel to the lengthwisedirection of the microchannels 110. A thin layer of neutron-absorbingmaterial 104, a thin layer of semiconducting material 106, and a thinlayer of electron emissive material 108 are formed on the surfaces ofthe microchannels 110 using, for example, atomic layer deposition.

The glass plate 102 is positioned between an input electrode 150 and anoutput electrode 152 (see FIGS. 4 and 5). The input and outputelectrodes 150, 152 can be metal layers (for example, nichrome) that arecoated onto the input and output surfaces of the glass plate 102. Theinput and output electrodes 150, 152 have openings that correspond tothe channel openings. The input and output electrodes 150, 152 arebiased at DC voltage levels configured to generate a high electric fieldin the microchannels 110. When an incident neutron interacts withneutron-absorbing material 104, the interaction produces moving chargedparticles that interact with the electron emissive material 108, whichin turn emits electrons that are attracted toward the end of themicrochannel 110 having a higher electric potential. As the electronsstrike against the channel walls, more electrons are released from theelectron emissive layer 108. From a single neutron, thousands ofelectrons emerge from the microchannel plate 100. The electrons can beelectronically captured as a signal pulse or allowed to strike aphosphor screen for visual imaging.

By using a glass plate 102 without lead or other high mass components,there will be a lower probability of false detection of neutrons due toreactions between ambient gamma rays and lead or other high masscomponents, as may occur in a conventional microchannel plate that usesglass with lead or other high mass components. Reducing the sensitivityof the microchannel plate 100 to gamma rays can increase thesignal-to-noise (S/N) ratio of neutron detection, leading to highercontrast radiography and detection of smaller features using neutronimaging. The insensitivity of the microchannel plate 100 to gamma raysalso allows lower level detection of clandestine nuclear materials, suchas plutonium. For example, by using low-Z glass, the gamma sensitivitycan be reduced by a factor of about 10 (compared to use of leadedglass), to approximately 0.1%.

The glass plate 102 can also use glass with lead or other high masscomponents, such as commercially available glass designed formicrochannel plates, available from PhotonisUSA, Sturbridge, Mass. Whensuch glass material is used, there is a probability of false detectionof neutrons due to reactions between ambient gamma rays and the lead orother high mass components. False detection can be reduced by using acoincidence technique or an induced pulse technique, described in moredetail below. In some examples, the glass may have more than 1 mol % andless than 25 mol % of lead or other element having an atomic numbergreater than 34.

To fabricate the microchannel plate 100, a glass plate 102 having asubstantially regular, parallel array of microscopic channels 110 isfabricated. Atomic layer deposition thin film techniques are used tomodify the surfaces of the microscopic channels 110 by forming asequence of functional layers that conformally coat the surfaces of theglass plate 102, including the surfaces of the microchannels 110.

In some implementations, a neutron-absorbing layer 104 of boron-10compound, such as boron-10 oxide (¹⁰B₂O₃), boron-10 nitride (¹⁰BN), orother material, having a thickness of about 1 μm, is formed on thesurface of the microchannels 110 by atomic layer deposition and servesas heavy charged particle and prompt gamma ray-emitting medium. Theboron-10 enriched layer can have a thickness within a range of, forexample, approximately 0.1 μm to 5 μm, or preferably from 0.5 μm to 5μm.

When a boron-10 atom 140 captures a neutron 142, the boron-10 nucleusfissions into an alpha particle (⁴He) 144 and a lithium-7 (⁷Li) particle146 traveling in opposite directions, as in the reaction below:

n+¹⁰B→⁷Li+⁴He+Q,

where Q is the energy released in the reaction. There is about 94%probability that the lithium-7 ion will initially be in an excitedstate, upon which the lithium-7 ion decays to a lower energy state andemits a 478 keV gamma ray. In some implementations, as described below,a coincidence technique or an induced pulse technique can be used todifferentiate the prompt emission of the gamma ray from background gammarays to increase the accuracy of the neutron detection.

A resistive (semiconducting) coating 106 is deposited by atomic layerdeposition on top of the neutron-absorbing layer 104 to establish anelectric field gradient and simultaneously allow a small leakage or biascurrent to flow through the microchannel surface, neutralizing thepositive surface charge due to emission of secondary electrons that formthe detectable electron amplification pulse following an electronavalanche. In some examples, the semiconducting layer can have athickness of about 100 nm. For example, the layer 106 can include aAlZn_(x)O_(y) alloy film. Other materials can also be used. Thesemiconducting layer 106 can have a thickness in a range from, forexample, 25 nm to 1000 nm, or preferably from 50 nm to 1000 nm.

A thin layer 108 of material having a high secondary electron yieldcoefficient (SEC) is deposited by atomic layer deposition on top of thesemiconducting coating 106 to enhance the overall gain. For example, thelayer 108 can include aluminum oxide (Al₂O₃) or magnesium oxide (MgO).Other materials can also be used. The electron emissive layer 108 canhave a thickness in a range from, for example, 3 nm to 20 nm, or from 3nm to 12 nm, or preferably 5 nm to 10 nm.

In some implementations, the atomic layer deposition can be a thin filmgrowth technique that uses alternating, saturating reactions betweengaseous precursor molecules and a substrate to deposit films inlayer-by-layer fashion. By repeating this reaction sequence in an ABAB .. . pattern, films from atomic monolayers to layers that are severalmicrons thick can be deposited with high and controlled precision.

In some implementations, the boron-10 oxide (or other compoundcontaining boron-10) can be replaced by gadolinium oxide (Gd₂O₃) ofabout the same thickness. Gadolinium can capture neutrons as in thefollowing reactions:

n+¹⁵⁵Gd→¹⁵⁶Gd+gamma rays+beta particles+x-rays (29 keV to 182 keV),

n+¹⁵⁷Gd→¹⁵⁸Gd+gamma rays+beta particles+x-rays (39 keV to 199 keV).

The beta particles (energetic electrons) can cause secondary electronemissions in the microchannels 110. In some implementations, thecoincidence technique or the induced pulse technique can be used todifferentiate the gamma rays (generated from the neutron-gadoliniuminteraction) from background gamma rays to increase the accuracy of theneutron detection.

In some implementations, the layer 104 includes alternating monolayersof boron-10 and gadolinium.

In some implementations, the layer 104 can include, for example, Li₂O,LiCO₃, or other compounds containing lithium-6. Lithium-6 can captureneutrons as in the following reaction:

⁶Li→⁴He+³H+4.78 MeV.

When a boron-10 particle 140 captures a neutron 142 and generates alithium-7 particle 146 and a helium-4 particle 144, one or both of thelithium-7 and helium-4 particles pass out of the glass substrate andenter one or more adjacent channels 110, in most cases freeing several(for example, 10 to 100) electrons 148 along the way. For example, othercharged particles, such as protons, hydroxide ions (OH⁻), cesium ions(the glass plate 102 may include cesium oxide) may also be freed by thelithium-7 and helium-4 particles, emerge from the glass walls and enterinto the channels 110.

In some implementations, the input electrode 150 may be connected to avoltage source to have a voltage level of −2000 to −1000 volts, and theoutput electrode 152 may be connected to a voltage source to have avoltage level of −100 volts. The DC voltage difference between the inputand output electrodes 150, 152 generates an electric field (e.g., about1 kV/mm) that attracts the free electrons toward the output electrode152. As the electrons strike against the channel walls, more electronsare released, triggering an avalanche of secondary electrons in themicrochannels 110 that can be detected using a signal read out from theoutput electrode 152.

In some examples, the neutron-absorbing layer 104 has a highconcentration of gadolinium or boron-10 atoms, such that the overallamount of gadolinium or boron-10 atoms in the neutron-absorbing layer104 can be comparable to (or higher than) a conventional microchannelplate in which the gadolinium or boron-10 atoms are doped within thebody of the glass plate. Thus, an incident neutron would just as likelyto strike a gadolinium or boron-10 atom in the neutron-absorbing layer104, compared to striking a gadolinium or boron-10 atom in the glassplate of the conventional microchannel plate.

In the case where the neutron-absorbing layer 104 contains lithium-6atoms, the overall amount of lithium-6 atoms in the neutron-absorbinglayer 104 can be higher than the amount of lithium-6 atoms that canpractically be doped within the body of the glass plate. This is becauseelevated levels of lithium in glass (e.g., more than a few mole percent)may result in undesirable devitrification or crystallization of theglass upon cooling from the melt. This problem does not occur (or is notsignificant) when lithium-6 is contained in a thin layer 104 on the wallof the microchannels 110.

By using thin layers of boron-10, gadolinium, or lithium-6 on thesurfaces of the microchannels 110, the sensitivity of neutron detectioncan be improved, compared to the conventional microchannel plate thathas boron-10, gadolinium, or lithium-6 distributed throughout the bulkor body of the glass plate. The improvement in sensitivity can be due toan increased concentration of boron-10 or gadolinium atoms, as well asreduced self-absorption of the alpha particle or lithium nucleus (in thecase of boron-10), conversion electron (in the case of gadolinium) oralpha particle and triton (in the case of lithium-6). When a neutron iscaptured by a boron-10, lithium-6, or gadolinium atom inside the glasssubstrate, the resulting helium-4, lithium-7 particle, triton, orconversion electron needs to enter the microchannel 110 in order togenerate a cascade of secondary electrons that can be detected at theoutput electrode 152.

In the conventional microchannel plate, the interaction between theneutron and boron-10 or lithium-6 (or gadolinium) atom may occuranywhere inside the glass substrate, for example, a location near themiddle between two adjacent microchannels. By comparison, in themicrochannel plate 100 of FIG. 1, the interaction between the neutronand boron-10 atom occurs in the neutron-absorbing layer 104. Thelikelihood of at least one of the resultant particles (helium-4 orlithium-7) escaping from the neutron-absorbing layer 104 and entering anadjacent microchannel 110 in the microchannel plate 100 can be higherthan that of the conventional microchannel plate.

In some implementations, the bulk of the glass plate 102 can use glassdoped with neutron sensitive material, such as boron-10, lithium-6and/or gadolinium. For example, the glass can have 0 to 20 mol % ofneutron sensitive material. A layer of neutron sensitive material 104 isadditionally provided on the microchannel surface. Although the ¹⁰B, ⁶Liand/or Gd atoms in the bulk glass may not be as effective in neutronconversion as those in the layer 104 (due to the longer escape pathsrequired for the charged particle reactants to reach the microchannelsurfaces), the ¹⁰B, ⁶Li and/or Gd atoms in the bulk glass will notdiminish the neutron conversion capability of the layer 104, and mayeven slightly enhance the overall neutron detection sensitivity of themicrochannel plate.

A microchannel plate having a neutron-sensitive layer 104 of ¹⁰B, ⁶Li,¹⁵⁵Gd, and/or ¹⁵⁷Gd and/or their compounds are highly effective for cold(e.g., having an energy of about 0.005 eV or less) and thermal (e.g.,having an energy of about 0.005 eV to 0.025 eV) neutron detection. The¹⁰B, ⁶Li, ¹⁵⁵Gd, and/or ¹⁵⁷Gd atoms have relatively large neutroncross-sections in these energy regions. For detecting neutron havinghigher energy levels, such as neutrons having energy levels in theso-called epithermal energy region (about 0.025 eV to about 100 eV),other neutron-sensitive materials can be used.

In some implementations, a microchannel plate neutron detector having aneutron-absorbing layer 104 of hafnium-177 (¹⁷⁷Hf) and/or its compoundcan detect neutrons having energy levels in the range of about 1 eV toabout 2 eV. The ¹⁷⁷Hf atoms provide large epithermal neutroncross-sections at two different resonance energies (1.1 eV and 2.4 eV)from K-shell neutron capture, leading to a resonance-induced internalconversion (IC) process. The resulting internal conversion electrons canbe used to generate a detectable pulse in the microchannel plate neutrondetector.

Hafnium has 32 isotopes whose half-lives are known, with mass numbers154 to 185. Naturally occurring hafnium is a mixture of six isotopes andthey are found in the percentages shown: 174Hf (0.2%), 176Hf (5.3%),177Hf (18.6%), 178Hf (27.3%), 179Hf (13.6%) and 180Hf (35.1%). The mostabundant is 180Hf at 35.1%. The 177Hf isotope, which has a high neutroncross section at the 1.1 eV (34.6 kb) and 2.4 eV (75 kb) resonances, isthe relevant isotope for epithermal neutron detection. This isotope ispresent in a reasonable amount, in hafnium compounds able to be eithercoated onto microchannel walls (e.g., HfO), or doped into the bulkmicrochannel plate structure, in a sufficient concentration to allow asufficient number of conversion electrons for generating detectablemicrochannel plate output pulses.

For example, in the case of ¹⁷⁷Hf, the neutron capture process issimilar to the neuron capture process for ¹⁵⁵Gd and ¹⁵⁷Gd (except that¹⁷⁷Hf captures neutrons at higher energy levels), in which internalconversion electrons are produced. Based on the neutron capturecross-sections and conversion efficiencies, ¹⁷⁷Hf would be expected toprovide comparable electron conversion efficiency (about 50%) in the 1eV to 2 eV epithermal energy region. For ¹⁷⁷Hf, the neutron captureprocess simply occurs at a higher resonance energy. It is this higherenergy resonance in ¹⁷⁷Hf that can be used to detect epithermalneutrons. The ¹⁷⁷Hf isotope can be applied in the form of a distinctmicrochannel wall coating in the form of hafnium oxide, underlying thesemiconducting and emitting layer coatings at the channel surface, ordoped into the bulk microchannel plate glass, or both.

In addition to ¹⁷⁷Hf, other elements may also provide enhanced neutroncapture and conversion to electrons in the epithermal range. Theseelements include samarium (Sm), erbium (Er), neodymium (Nd), tantalum(Ta), lutetium (Lu), europium (Eu), dysposium (Dy), or thulium (Tm).These materials have different resonance energies and can provide highlyspecific options to aid detection of neutrons within targeted energybands that lie in the epithermal range, in which incorporating specificelements within the microchannel plate detector material allows specifictuning or “tailoring” of the microchannel plate detector response tocertain resonance energies. This may be useful in applications such asneutron resonance absorption imaging (NRAI).

For example, ¹⁴⁹Sm has a 20,600 b cross section at the 0.87 eV resonanceenergy. Here, the internal conversion process involving K-shell X-raysoccurs with 4% probability (resulting in somewhat lower neutrondetection efficiency generally than ¹⁷⁷Hf). Nevertheless, use of ¹⁴⁹Smcan still enhance microchannel plate detector sensitivity to neutrons,at least those having energies in the vicinity of this resonance.

The layer of neutron sensitive material 104 can include at least 30 mol%, or at least 50 mol % of neutron sensitive material, including atleast one the elements hafnium (Hf), samarium (Sm), erbium (Er),neodymium (Nd), tantalum (Ta), lutetium (Lu), europium (Eu), dysposium(Dy), or thulium (Tm). The layer of neutron sensitive material 104 caninclude a compound having at least one of hafnium (Hf), samarium (Sm),erbium (Er), neodymium (Nd), tantalum (Ta), lutetium (Lu), europium(Eu), dysposium (Dy), or thulium (Tm). For example, the compound can bein oxide form, or in certain cases, as a nitride. The layer of neutronsensitive material can have a thickness in a range from 0.1 to 5microns. The layer of semiconducting material can have a thickness in arange from 50 to 1000 nm.

Table 1 below shows epithermal neutron resonances and several stableparent and product nuclides. Gd is included in the table to show itseffectiveness for thermal neutron (having energy about 0.025 eV or less)capture and internal conversion to electrons.

TABLE 1 Resonance Cross K-shell X-rays (% Nuclide energy (eV) section(barns) neutron capture) 155Gd 0.0268 58,000 25 157Gd 0.0314 230,000 25180Ta 0.433 12,000 81 167Er 0.460 10,100 32 149Sm 0.872 20,600 3.7 177Hf1.098 34,600 41 177Hf 2.38 75,000 41

For many applications, it is useful for neutron detectors to maintainhigh neutron detection efficiency above the thermal energy region. Thishigher neutron energy region is sometimes loosely and broadly termed the“epithermal region,” which is sometimes further divided into severalranges, e.g., epithermal (0.025-0.4 eV), cadmium (0.4-0.6 eV),epicadmium (0.6-1 eV), slow (1 eV-10 eV), and resonance (10-300 eV)regions. It is useful that neutron detection efficiency be generallyenhanced in the neutron energy range between 0.025 eV to 1 keV.

In some implementations, the bulk of the glass plate 102 can use glassdoped with neutron sensitive material such as hafnium, samarium, erbium,neodymium, tantalum, lutetium, europium, dysposium, and/or thulium toprovide enhanced neutron capture and conversion to electrons in theepithermal range. For example, the glass can have up to 20 mol % ofneutron sensitive material. In some examples, the neutron sensitivematerial is provided in the bulk glass only. In some examples, a layerof neutron sensitive material 104 is additionally provided on themicrochannel surface. Although the hafnium, samarium, erbium, neodymium,tantalum, lutetium, europium, dysposium, or thulium atoms in the bulkglass may not be as effective in neutron conversion as those in thelayer 104 (due to the longer escape paths required for the chargedparticle reactants to reach the microchannel surfaces), the hafnium,samarium, erbium, neodymium, tantalum, lutetium, europium, dysposium, orthulium atoms in the bulk glass will not diminish the neutron conversioncapability of the layer 104, and may enhance the overall neutrondetection sensitivity of the microchannel plate.

FIG. 2 shows an AB sequence for the atomic layer deposition of amonolayer of a compound AB. Graph 120 shows a substrate 122 (such as theglass plate 102) having a surface 124 with discrete reactive sites(represented by notches 126). In cycle A, exposing the surface 124 toreactant A 128 results in self-limiting chemisorption of a monolayer 130of material A. Graph 134 shows that a monolayer 130 of material A hasbeen formed on top of the substrate 122. Graph 136 shows that thesurface 132 of the monolayer 130 of material A becomes the startingsurface for the deposition of material B. In cycle B, exposure tomolecules of material B 134 results in the surface 132 being coveredwith a monolayer 136 of material B. Consequently, one AB cycle depositsone monolayer of compound AB and regenerates the initial surface.

The alternating reaction strategy eliminates the “line of sight” or“constant exposure” requirements that limit conventional thin filmmethods such as physical- or chemical-vapor deposition, and offers thecapability to coat complex, three-dimensional objects with precise,conformal layers. This atom-by-atom growth approach allows atomic layerdeposition to apply precise and conformal coatings over the surfaces ofthe microchannels 110. Each individual AB cycle can be performed in, forexample, several seconds.

The following describes example reactions in which a layer of aluminumoxide (which can be used as the electron emissive layer 108 in FIG. 1A)is deposited in a binary reaction sequence (the surface species aredesignated by asterisks):

OH*+Al(CH₃)₃→O—Al(CH₃)₂*+CH₄   A)

O—Al—CH₃*+H₂O→O—Al—OH*+CH₄   B)

In reaction A, the glass substrate surface is initially covered withhydroxyl (OH) groups. The hydroxyl groups react with trimethyl aluminumvapor (Al(CH₃)₃, TMA) to deposit a monolayer of aluminum-methyl groupsand give off methane (CH₄) as a byproduct. Because TMA is inert to themethyl-terminated surface, further exposure to TMA yields no additionalgrowth beyond one monolayer. In reaction B, this new surface is exposedto water vapor regenerating the initial hydroxyl-terminated surface andagain releasing methane. The net effect of one AB cycle is to depositone monolayer of Al₂O₃ on the surface. Despite the atom-by-atom growthapproach, thick films can be applied efficiently on planar surfacessince the individual AB cycles can be performed in several seconds. Whencoating porous materials, the AB cycle time is increased to allow thegaseous precursors to diffuse into the nano-pores.

A layer of gadolinium oxide (Gd₂O₃) (for example, for use as the layerof neutron sensitive material 104 in FIG. 1A) can be prepared usingatomic layer deposition, with a variety of precursor combinationsincluding gadolinium tris(2,2,6,6-tetramethyl-3,5-heptanedione)(Gd(thd)₃) and ozone, gadolinium tris(methylcyclopentadiene) (Gd(MeCp)₃)and water, and gadolinium tris(bis-trimethylsilylamide) (Gd[N(SiMe₃)₂]₃)and water. Although the Gd(MeCp)₃ precursor does not strictly yieldself-limiting growth, this compound provides a relatively high atomiclayer deposition growth rate of up to 0.3 nm/cycle with acceptable filmthickness uniformity, and this can facilitate the deposition of the 1 μmneutron absorbing layer 104. Building up a layer 1 μm thick may requireapproximately 3,300 cycles. Evaluation of the coatings can be made byoptical microscopy or scanning electron microscopy.

The resistive (semiconducting) layer 106 can be formed using atomiclayer depositing techniques using a broad range of various materials.For example, alternating layers of two or more different materials canbe deposited such that the resulting physical, chemical, and electronicproperties of the compound material can be fine-tuned in between theseparate component properties. Atomic layer deposition can be used toprepare thin film coatings with controlled resistance by blendingtogether materials having both high and low resistance levels, such asAl₂O₃ and zinc oxide (ZnO) respectively. Al₂O₃ can be deposited usingalternating exposures to TMA and H₂O, and ZnO can be deposited usesalternating exposures to diethyl zinc (DEZ) and H₂O. By controlling therelative number of TMA/H₂O and DEZ/H₂O cycles, AlZn_(x)O_(y) alloy filmscan be deposited with precise control over both composition andthickness and the resistivity can be adjusted over a very wide rangefrom 10⁻³ to 10¹⁶ Ω-cm. By depositing conformal layers of theAlZn_(x)O_(y) alloy films on the surface of the glass plate 102, theresistance across the surface of the glass plate 102 can be controlled.For example, an AlZn_(x)O_(y) alloy film with a thickness of 100 nm mayyield a resistance across the glass plate of about 10⁷Ω.

The layer of neutron sensitive material 104 can be formed using atomiclayer depositing techniques. For example, to form a layer of B₂O₃,precursors BBr₃ and H₂O can be used. To form a layer of BN (boronnitride), precursors BBr₃ and NH₃ can be used. To form a layer ofsamarium oxide (Sm₂O₃), precursors Sm(N(Si(CH₃)₃)₂)₃ and C₂₇H₃₉Sm can beused. To form a layer of erbium oxide (Er₂O₃), precursorsEr(OCC(CH₃)₃CHCOC(CH₃)₃) and Er(C₅H₄C₄H₉)₃ can be used. To form a layerof neodymium oxide (Nd₂O₃), precursor material Nd(N(Si(CH₃)₃)₂)₃ can beused. To form a layer of tantalum pentoxide, mixed tantalum oxides, ortantalum nitride, precursors Ta(N(CH₃)₂)₅, Ta(OC₂H₅)₅,(CH₃)₃CNTa(N(C₂H₅)₂), and/or C₁₃H₃₃N₄Ta can be used.

FIG. 3 is a diagram showing microchannels 110 in the glass plate 102.

FIG. 4 is a diagram showing the development of an avalanche of secondaryelectrons. In this example, the neutron-sensitive layer 104 includesboron-10, but similar principles apply to other examples in which theneutron-sensitive layer 104 includes lithium-6, gadolinium, hafnium,samarium, erbium, neodymium, tantalum, lutetium, europium, dysposium,and/or thulium. Each channel 110 can serve as an independent electronmultiplier. In operation, when an incident neutron 142 strikes the glassplate 102, the neutron 142 is captured by a boron-10 atom, and an alphaparticle (⁴He) and a lithium-7 (⁷Li) particle are released. One or bothof the lithium-7 and helium-4 particles pass out of the glass wall andenter one or more adjacent channels 110, freeing electrons along theway. Concurrently, the DC bias voltage is applied between the input andoutput electrodes 150, 152 such that the output electrode 152 has a morepositive DC bias voltage than the input electrode 150. The DC biasvoltage generates an electric field (for example, about 1 kV/mm) thatattracts free electrons toward the output electrode 152. As freeelectrons strike the channel walls, more electrons (for example,secondary electrons 154) are released to form a cascade of electrons 156that exit the output surface of the glass plate 102 and is detected as asignal at the output electrode 152. Thus, the glass plate 102 acts as anelectron multiplier. In some implementations, the glass plate 102 canuse commercially available glass material designed for use inmicrochannel plates, such as glass material available from PhotonisUSA,Sturbridge, Mass.

When the layers 104, 106, and 108 are formed on the glass plate 102, thelayers 104, 106, 108 will not only be coated on the walls of themicrochannels 110, they will also be coated on the input surface andoutput surface (web area) of the glass plate 102. Having the layers 104,106, and 108 on the input and output surfaces of the glass plate 102will likely have a small or negligible effect on the sensitivity ofneutron detection. The effect, if any, can be compensated for byslightly increasing the microchannel plate bias voltage.

The following describes a neutron detector that uses the coincidencetechnique. This technique can be used when the microchannel plate 100includes a neutron absorbing particle in which an interaction between aneutron and the neutron absorbing particle generates one or more gammarays. The coincidence technique is described in U.S. Pat. No. 7,439,519,“Neutron Detection Based on Coincidence Signal,” herein incorporated byreference.

Referring to FIG. 5, in some implementations, a neutron detector 170includes a microchannel plate detector 172 and a gamma ray detector 174.The signal from the microchannel plate detector 172 is sent to a signalprocessor, such as a coincidence unit 188, for comparison with a signalfrom the gamma ray detector 174. The microchannel plate detector 172includes a microchannel plate 100 having a glass plate 102 positionedbetween an input electrode 150 (connected to a more negative voltage)and an output electrode 152 (connected to a more positive voltage). Ananode 160 is provided to collect electron emissions from themicrochannel plate 100. The microchannel plate 100 and the anode 160 arehoused within a vacuum chamber 176. The microchannel plate 100 issensitive to neutrons (due to a layer of neutron sensitive material104). The bulk material of the glass plate 102 may also absorb gammarays and generate photoelectrons that enter the channel to generatespurious noise pulses. A readout signal from the anode 160 (or theoutput electrode 152) may indicate detection of a neutron or a gamma raybut typically does not provide information on whether a neutron or agamma ray is detected.

In some implementations, the gamma ray detector 174 includes a fastscintillator crystal 178 and a photomultiplier tube (PMT) 180. Thescintillator crystal 178 emits scintillation light upon receiving agamma ray, and the PMT 180 captures the scintillation light. Forexample, the scintillator crystal 178 can be a LaBr₃:Ce scintillatorcrystal, BrilLianCe®380 crystal, from Saint-Gobain Crystals, Newbury,Ohio. The PMT 180 can be model 83112-502, available from BurleIndustries.

As described above, when a boron-10 atom captures a neutron to generatea lithium-7 ion, there is about 94% probability that the lithium-7 ionwill initially be in an excited state, upon which the lithium-7 iondecays to a lower energy state and emits a 478 keV gamma ray. When agadolinium atom (either ¹⁵⁵Gd or ¹⁵⁷Gd) captures a neutron, gamma raysare also emitted. Other neutron sensitive materials that generate gammarays upon capture of neutrons can also be used. If both the prompt gammaand the neutron pulse are observed within a short coincidence window(for example, about 10 ns), then the neutron event can be positivelytagged with high confidence. All other counts outside this timing windoware rejected, and considered as random gammas. The timing coincidencewindow of about 10 ns is short enough to statistically exclude mostbackground gamma rays (even with gamma flux rates in the MHz region).

By measuring a time proximity of a signal from the microchannel plate100 and a signal from the PMT 180, one can determine whether a neutronhas been detected by the microchannel plate detector 172. The signalfrom the microchannel plate 100 indicates detection of a neutron or agamma ray. The signal from the PMT 180 indicates detection of a gammaray. Detecting a signal from the PMT 180 shortly (e.g., within 0.1 to 10ns) after detecting a signal from the microchannel plate 100 indicates ahigh likelihood that a neutron absorption event accompanied by gamma rayemission has occurred.

An advantage of the neutron detector 170 is that by detectingcoincidence between signals from the microchannel plate detector 172 andthe gamma ray detector 174, false positive detection of neutrons can bereduced significantly.

The output signal of the microchannel plate detector 172 is sent to anamplifier 182 to amplify the signals received at the anode 160 (or theoutput electrode 152) of the microchannel plate detector 172, and theoutput of the amplifier 182 is sent to a timing module 184. The outputof the PMT 180 is sent to a timing module 186. The timing modules 184and 186 condition the signals from the microchannel plate detector 172and PMT 180, taking into consideration the different signal pathstraveled by the signals from the microchannel plate detector 172 and PMT180 to the coincidence unit 188. The outputs of the timing modules 182and 184 are sent to the coincidence unit 188, which determines whetherthe signal from the timing module 186 occurs within the timingcoincidence window (e.g., 0.1 to 10 ns) of the signal from the timingmodule 184. The coincidence unit 188 can be, for example, model 2040,from Canberra, Meriden, Conn.

The coincidence unit 188 determines a time difference between a signalreceived from the timing module 184 and a later signal received from thetiming module 186, and compares the time difference with the presettiming coincidence window. If the time difference is less than thetiming coincidence window (for example, 10 ns), the coincidence unit 188generates a pulse that is sent to a scaler/counter 190, indicating aneutron event. The counter 190 can be configured to count the number ofneutron events per unit of time (for example, second). The counter 190can be, for example, model 512, from Canberra. The output signal of thecounter 190 can be sent to a computer 192 or data acquisition device forrecording and analysis of the signal.

If there is no coincidence within 10 ns between the output signals ofthe microchannel plate detector 172 and the gamma ray detector 174, theneither (i) a gamma ray of arbitrary energy is detected by themicrochannel plate detector 172, and no 478 keV gamma ray is detected bythe scintillator 178 within the 10 ns timing window, or (ii) a gamma rayis detected by the scintillator 178 but no corresponding neutron signalis detected by the microchannel plate detector 172.

The probability that a 478 keV gamma ray is detected within a 10 nstiming window, and another background gamma ray of any energy beingdetected by the microchannel plate detector 172 (which has 0.2% to 2%detection efficiency to gamma rays), is very small. Because there isabout 94% probability that the boron-10 and neutron reaction willgenerate a lithium-7 ion in the excited state that decays with anemission of a 478 keV gamma ray, there is a probability of about 6% thatneutron events would not result in an emission of a 478 keV gamma ray.Of the 478 keV gamma rays that are emitted isotropically, about 16% canbe detected by the scintillator crystal 178 that is placed on the outputside of the microchannel plate detector 172. Using two largerscintillator crystals, one positioned at the input side and the otherpositioned at the output side of the microchannel plate detector 172,can significantly increase the detection rate of the gamma rays, due tolarger solid angle capture.

The neutron detector 170 can have a spatial resolution of about 10 μm,and with good detection capabilities for both cold and thermal neutrons.The neutron detector 170 can have a wide variety of applications inneutron radiography and scattering, including biological/medicalimaging, inspection of electronic components, construction materials,soils, foods, art objects, studies of hydrogen and water content,analysis of moisture transport and diffusion, and analysis of theaccumulation of hydrogen in irradiated samples as fuel element claddingand spallation target studies. The neutron detector 170 can be used inhand-held imaging and counting for monitoring and imaging of nuclearreactor leaks, larger arrayed 2-D position-sensitive detection systemsfor neutron scattering halls, neutron powder diffraction, andpotentially real-time neutron imaging.

For example, the neutron detector 170 can provide a better insight offuel cell development by monitoring the presence of water in the fuelcell membrane and preventing its unwanted occurrence. This can lead tothe enhancement for fuel cell products. The neutron detector 170 can beused in the study of structural biology and biological physics, such asobtaining information about the structure and dynamics of biologicalsystems. Neutron imaging techniques have an advantage in probing deeplyinto the sample volume or bulk. As an example, changes in neutron energyand momentum, when scattered at small angles from proteins,macromolecular crystal lattices, or membranes, can yield fine structuraldetails. The respective deBroglie wavelengths of cold and thermalneutron beams, of greater than 4 Å and 2 Å, with negligible absorptionin biological materials, correspond to atomic spacing and fluctuationamplitudes as well as energies that are well-suited to typicalexcitation energies in biological samples. The neutron detector 170 canbe used in airport portal screening, monitoring of special nuclearmaterials (SNM), first responders, and critical operations.

The following describes a neutron detector that uses the induced pulsetechnique. The induced pulse technique is described in U.S. Pat. No.8,445,861, “Neutron Detection Based on Induced Charges,” filed on Jan.28, 2011, herein incorporated by reference.

In some implementations, neutrons and gamma rays can be distinguishedbased on characteristics of charges induced on input and outputelectrodes of the microchannel plate 100. When an incident neutron or agamma ray interacts with the glass plate 102, the interaction producesmoving charged particles that induce electric charges in the input andoutput electrodes 150, 152. Due to different energy levels of theparticles generated from the interactions, the electric charges inducedon the input and output electrodes 150, 152 for an incident neutron aredifferent from those for an incident gamma ray. By using informationabout the charges induced on both the input and output electrodes 150,152, one can detect neutrons with high confidence.

Referring to FIG. 6, a neutron detector 250 includes a microchannelplate 100 have an input electrode 150, an output electrode 152, and aglass plate 102 positioned between the input and output electrodes 150,152. The microchannel plate 100 has an array of microscopic channels 110each having a diameter of about, for example, 5 to 10 microns (μm). Thesurface of the microchannels 110 may have a thin layer ofneutron-absorbing material 104, a thin layer of semiconducting material106, and a thin layer of electron emissive material 108 formed using,for example, atomic layer deposition. The neutron-absorbing material canbe, for example, compounds that contain boron-10 or lithium-6. Theneutron-absorbing material can be, for example, compounds that containhafnium, samarium, erbium, neodymium, tantalum, lutetium, europium,dysposium, and/or thulium. The input and output electrodes 150, 152 canbe, for example, metal layers that are coated onto the top and bottomsurfaces of the glass plate 102. The input and output electrodes 150,152 have openings that correspond to the channel openings.

In some implementations, the input electrode 150 may be connected to avoltage source to have a voltage level of −2000 to −1000 volts, and theoutput electrode 152 may be connected to a voltage source to have avoltage level of −100 volts. The DC voltage difference between the inputand output electrodes 150, 152 generates an electric field (e.g., about1 kV/mm) that attracts the free electrons toward the output electrode152. As the electrons strike against the channel walls, more electronsare released, triggering an avalanche of secondary electrons in themicrochannels 110. The electrons pass through the openings in the outputelectrode 152 and are collected by an anode collector 252, which isconnected to the electric ground voltage through a pad 254. The pad 254may be connected to auxiliary instruments, such as an oscilloscope.

The glass in the microchannel plate 102 may include lead (Pb), which issensitive to gamma rays. When an incident gamma ray interacts with alead atom, an energetic electron is released, which passes out of theglass and enter an adjacent channel, in most cases freeing a fewelectrons (e.g., less than 10) along the way. The one or a few electronsare attracted toward the output electrode 152, and as the electronsstrike against the channel walls, more electrons are released,triggering an avalanche of secondary electrons in the microchannels 110.The electrons pass through the openings in the output electrode 152 andare collected by the anode 252.

The avalanche of secondary electrons in the microchannels 110 can inducecharges on the input electrode 150 and the output electrode 152. Thecharges induced on the input electrode 150 can be amplified by anamplifier 256. A capacitor 258 between the input electrode 150 and theamplifier 256 blocks DC signals while allowing AC signals to pass,decoupling the amplifier 256 from the input electrode 150. A resistor260 is provided to prevent voltage spikes from damaging the amplifier256. The output signals from the amplifier 256 can be conditioned by asignal conditioning unit 262. The charges induced on the outputelectrode 152 can be amplified by an amplifier 264. A capacitor 266between the output electrode 152 and the amplifier 264 blocks DC signalswhile allowing AC signals to pass, decoupling the amplifier 264 from theoutput electrode 152. A resistor 268 is provided to prevent voltagespikes from damaging the amplifier 264. The output signals from theamplifier 264 can be conditioned by a signal conditioning unit 270. Forexample, the signal conditioning units 262, 270 may introduce delays inone or both of the output signals from the amplifiers 256, 264 tocompensate for differences in the signal paths traveled by the signals.After amplification and conditioning, the signals from the input andoutput electrodes 150, 152 are sent to a high-speed dual channelanalog-to-digital converter 272 that samples the analog signals andconverts them to digital data. The digital data are stored and processedby a computer 274 (or any other data processing apparatus).

More electrons are generated from the interaction between a neutron andthe boron-10 or lithium-6 atom than from the interaction between a gammaray and a lead atom. As a result, charges Q_(in) ^(α) and Q_(out) ^(α)induced on the input and output electrodes 150, 152, respectively,associated with a neutron event are greater than the charges Q_(in) ^(γ)and Q_(out) ^(γ) induced on the input and output electrodes 150, 152,respectively, associated with a gamma ray event.

Judging by the amplitude of the induced charges in the input and outputelectrodes 150, 152 may not be sufficient to accurately determinewhether a neutron or a gamma ray has been detected because the totalnumber of secondary electrons depends on the location of the interactionbetween the neutron and the boron-10 or lithium-6 atom, or between thegamma ray and the lead atom. The difference in the induced chargesresulting from a neutron interacting with a boron-10 or lithium-6 atomlocated near the output electrode 152 and the induced charges resultingfrom a gamma ray interacting with a lead atom located near the inputelectrode 150 may not be sufficiently large to allow accuratedetermination of whether a neutron or gamma ray is detected.

In some implementations, a neutron verification parameter W1 used fordiscriminating between neutrons and gamma rays can be calculated usingthe following formula:

$\begin{matrix}{{{W\; 1} = \frac{Q_{in}/Q_{out}}{Q_{in} + Q_{out}}},} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

where Qin represents the charges induced on the input electrode 150 andQout represents the charges induced on the output electrode 152.

In Equation 1, the charges are normalized to compensate for thevariances in the induced charges due to the variations in the locationswhere the interactions occur. The denominator (Qin+Qout) will besubstantially larger for a neutron event than a gamma ray event, so theneutron verification parameter W1 will have a substantially smallervalue for a neutron event than for a gamma ray event.

The neutron detector 250 can be calibrated by irradiating themicrochannel plate 100 with neutrons generated from a neutron source andcalculating the values for the neutron verification parameter W1. Theneutron verification parameters W1 may have values that fall within afirst range of values. The first range of values are stored in thecomputer 274 and are associated with neutron events.

The microchannel plate 100 is then irradiated with gamma rays generatedfrom a gamma ray source, and values for the neutron verificationparameter W1 are calculated. The neutron verification parameters W1 mayhave values that fall within a second range of values. The second rangeof values are stored in the computer 274 and are associated with gammaray events.

When the neutron detector 250 is used to detect radiation from anunknown source, the charges Qin and Qout induced on the input and outputelectrodes 150, 152, respectively, are measured and the neutronverification parameter W1 is calculated. If the neutron verificationparameter W1 is within the first range of values associated with aneutron event, the neutron detector 250 generates a signal indicatingthat a neutron has been detected. If the neutron verification parameterW1 is within the second range of values associated with a gamma rayevent, the neutron detector 250 determines that a gamma ray has beendetected.

In some implementations, charges induced on the input and outputelectrodes 150, 152 due to movements of the charges prior to thedevelopment of avalanche of secondary electrons can also be used todiscriminate between a neutron event and a gamma ray event.

In some implementations, by using information about the polarities ofcharges induced on the output electrode, or both the input and outputelectrodes, one can detect neutrons with high confidence and rejectaccompanying gamma ray interference. This pulse polarity technique isdescribed below and also described in U.S. patent application Ser. No.13/842,904, filed on Mar. 15, 2013, herein incorporated by reference inits entirety.

Referring to FIG. 10, a neutron detector 300 includes a microchannelplate 302 have an input electrode 304 (also referred to as a frontelectrode), an output electrode 306 (also referred to as a rearelectrode), and a glass plate 308 positioned between the input andoutput electrodes 304, 306. The input electrode 304 is positioned at theside of the microchannel plate where neutrons and gamma rays are inputto the microchannel plate. The glass plate 308 has an array ofmicroscopic channels each having a diameter of about, for example, 5 to10 microns (μm). Each channel can have, for example, a circular, square,rectangle, or hexagon cross sectional shape. Each channel serves as anindependent electron multiplier and has an inner wall surface formed ofa semi-conductive and electron emissive layer. The glass plate 308 canhave a thickness of, e.g., about 1 mm. The input and output electrodes304, 306 can be, for example, metal layers that are coated onto the topand bottom surfaces of the glass plate 308. The input and outputelectrodes 304, 306 have openings that correspond to the channelopenings.

The terms “top” and “bottom” refer to the relative positions of surfacesof the MCP 302 when the MCP 302 is oriented such that the surfaces ofthe MCP are horizontal, and incoming radiation enters the microchannelsfrom the top. The MCP 302 can be used in various orientations and invarious positions relative to a radiation source such that what we referto as the “top surface” may actually be positioned below the “bottomsurface.”

In some implementations, the microchannel plate 302 is similar to themicrochannel plate 100 of FIG. 1, in which the microchannel plate 302includes a layer of neutron sensitive material(s), such as boron-10(¹⁰B) and/or lithium-6 (⁶Li), formed on the surface of each of themicrochannels. The layer of neutron sensitive material(s) can alsoinclude gadolinium, hafnium, samarium, erbium, neodymium, tantalum,lutetium, europium, dysposium, and/or thulium, or a compound thatincludes any of the above elements.

In some implementations, the input electrode 304 may be connected to avoltage of about −2000 to −1000 volts, and the output electrode 306 maybe connected to a voltage of about −100 volts. The voltages can begenerated by a high-voltage power supply 310 and a voltage divider 312having series connected resistors 334 and 336. The DC voltage differencebetween the input and output electrodes 304, 306 generates an electricfield (e.g., about 1 kV/mm) that attracts the free electrons toward theoutput electrode 306. As the electrons strike against the channel walls,more electrons are released, triggering an avalanche of secondaryelectrons in the microchannels. The electrons pass through the openingsin the output electrode 306 and are collected by an anode collector 314.

The input electrode 304 is connected to a first high pass filter 316that attenuates low frequency signals and allows high frequency signalsto pass to a pre-amplifier 318 and an amplifier 320, which amplify thesignal from the input electrode 304 and sends the amplified signal to anoscilloscope 322. For example, the high frequency signals can refer tosignals having periods in a range from about 10 to 100 microseconds, andthe low frequency signals can refer to signals having frequencies thatare less than 100 Hz. The output electrode 306 is connected to a secondhigh pass filter 324 that attenuates low frequency signals and allowshigh frequency signals to pass to a pre-amplifier 326 and an amplifier328, which amplify the signal from the output electrode 306 and sendsthe amplified signal to the oscilloscope 322. The anode 314 is connectedto a pre-amplifier 330 and an amplifier 332, which amplify the signalfrom the anode 314 and sends the amplified signal to the oscilloscope322.

The glass plate 308 may include a proportion of elements with medium orhigher atomic number (e.g., Z>˜10), which increase the tendency forgamma ray absorption and charged particle conversion. For example, whenan incident gamma ray interacts with a high Z lead atom (Z=82), anenergetic photoelectron is released, which passes out of the glass andenters an adjacent hollow channel, in most cases liberating a fewsecondary electrons (e.g., less than 10) from the channel walls. The oneor a few electrons are attracted toward the output electrode 306, and asthe electrons strike against the channel walls, more electrons arereleased, triggering an avalanche of secondary electrons in themicrochannels. The electrons pass through the openings in the outputelectrode 306 and are collected by the anode 314. Such gamma ray-inducedevents may be confused with neutron events.

The avalanche of secondary electrons in the microchannels can occurwithin a few nanoseconds or less of the occurrence of a neutron event(meaning that a neutron is captured by the microchannel plate 302) or agamma ray event (meaning that a gamma ray is captured by themicrochannel plate 302). Due to the strong electric field (˜1 kV/mm)between the input and output electrodes, the secondary electrons travelexceedingly fast inside the evacuated microchannels. Thus, upon aneutron or gamma ray event, a pulse signal can be detected at the anode314 within about several nanoseconds or less. The pulse in an anodesignal 338 is a result of the secondary electron avalanche inducing anegative-going charge pulse at the anode 314, so the anode pulse istypically a negative pulse. In this description, a negative pulse meansthat the pulse peak is negative relative to a baseline voltage, which isthe voltage level at the anode 314 when there is no neutron or gamma rayevent.

After the detection of the avalanche of secondary electrons at the anode314, another set of signals can be detected at the input electrode 304,the output electrode 306, and the anode 314. As described in more detailbelow, after a gamma ray event, a positive pulse can be detected at theinput electrode 304 and a positive pulse can also be detected at theoutput electrode 306. After a neutron event, a positive pulse can bedetected at the input electrode 304, regardless of pulse integrationtime and a negative pulse can be detected at the output electrode 306,depending upon pulse integration time. In this description, a positivepulse means that the pulse peak is positive relative to the baselinevoltage, regardless of pulse integration time. The baseline voltage forthe input electrode 304 is the voltage level at the input electrode 304when there is no neutron or gamma ray event. The baseline voltage forthe output electrode 306 is the voltage level at the output electrode306 when there is no neutron or gamma ray event. The positive andnegative pulses on the electrodes occur, e.g., within about 5 to 50 μsafter occurrence of the neutron or gamma ray events. These inducedpulses are due, not to the secondary electron avalanche inside thehollow microchannel, but in a qualitatively different way, to themovement of a large bulk ionization charge or charge clusters, withinthe solid channel wall material, the motion of which induces charge onthe MCP metal electrode.

In some implementations, by observing the pulses at the output electrode306, it is possible to determine whether a neutron or some otherradiation or particle has been detected. If the signal at the outputelectrode 306 is negative, integrated over a certain time period, thereis a high probability that a neutron has been detected. If the signal atthe output electrode 306 is positive, integrated over a comparable timeperiod, there is a high probability that something other than a neutron,such as a gamma ray, ion, electron, ultra-violet ray, X-ray, has beendetected.

In some implementations, the pulse signal at the output electrode 306 isintegrated over a period of time, such as several microseconds, or about1 to 50 μs, and the sign of the integral is used to determine whether aneutron is detected. If the integral is negative, there is a highprobability that a neutron has been detected. If the integral ispositive, it is likely that something other than a neutron, such as agamma ray or ion, electron, ultra-violet ray, X-ray, has been detected.

In some implementations, the signal at the output electrode 306 may beinverted and amplified, such that a positive pulse is detected after aneutron event and a negative pulse is detected after a gamma ray event.Similarly, in some implementations, the signal at the output electrode306 may be inverted and integrated over a period of time, such that apositive integral is generated after a neutron event and a negativeintegral is generated after a gamma ray event.

A calibration of the neutron detector 300 is performed in which a knownneutron source is used to provide neutrons, and the time-integratedsignal at the output electrode 306 is measured to determine whether itis positive or negative. A known source of gamma rays is used to providegamma rays, and the signal at the output electrode 306 is measured todetermine whether it is positive or negative. The calibration resultsare stored. Afterwards, detection of a neutron or a gamma ray can bedetermined based on the calibration data.

In some implementations, by observing the pulses at both the inputelectrode 304 and the output electrode 306, it is possible to determinewhether a neutron or some other radiation or particle has been detected.When a gamma ray is detected, the integral of the signal at the outputelectrode 306 integrated over a specified period of time has a polaritythat is the same as the polarity of the integral of the signal at theinput electrode 304 integrated over a comparable period of time. When aneutron is detected, the integral of the signal at the output electrode306 integrated over a specified period of time has a polarity that isopposite to the polarity of the integral of the signal at the inputelectrode 304 integrated over a comparable period of time.

In some implementations, the pulse signal at the input electrode 304 isintegrated over a period of time (such as several microseconds, or about1 to 50 μs) to generate a first integral, and the pulse signal at theoutput electrode 306 is integrated over the period of time to generate asecond integral. When a gamma ray is detected, the first integral has apolarity that is the same as the polarity of the second integral. When aneutron is detected, the first integral has a polarity that is oppositeto the polarity of the second integral.

The signals from the amplifier 320, 328, and 332 can be sent to a signalanalyzer that can automatically analyze the polarity of the signals,perform integration of the signals, analyze the polarities of theintegrated signals, compare polarities of signals at the input andoutput electrodes, and compare the polarities of the integrated signalsat the input and output electrodes in order to determine whether aneutron has been detected.

FIG. 7 is a diagram of a process 200 for fabricating a microchannelplate.

In process 200, a structure that defines a plurality of microchannels isfabricated 202. For example, the structure can be the glass plate 102 ofFIG. 1A. The structure can be formed by using, for example, fibers eachincluding a soluble core and a layer of cladding surrounding the solublecore, and removing the soluble core to form microchannels. The fiberscan be formed as follows. Low-Z bulk glass that includes zero or a lowpercentage of lead (e.g., having less than 5 mol %) and zero or a lowpercentage of neutron sensitive material (e.g., having less than 1 mol%) is melted and cast into right cylinders and reworked into a bulkglass tube by high-temperature extrusion through a set of dies. Solublecore glass is made into a rod having a diameter of, for example, oneinch. The inner diameter of the bulk glass tube is slightly larger thanthe diameter of the core glass rod. The core glass rod is inserted intothe bulk glass tube. A combination of the rod and the glass tube isheated and drawn into a pencil sized rod (e.g., the combination ispulled to become longer and have a smaller diameter). A series of drawnrods are bundled, heated, and drawn into a fiber such that the initial1-inch diameter core rod is reduced to 5 μm in diameter.

The drawn fibers are assembled in a hexagonal preform and fused togetherforming a solid glass billet. The billet is sliced into thin wafers at a0.5-1° bias (for accurate neutron event localization and minimalparallax blurring). The faces of the thin wafer are polished, and theedges of the wafer are ground into a round shape to produce a solidmicrochannel plate blank. The solid microchannel plate blank is immersedinto a dilute acid to etch away the core glass, leaving millions ofsmall holes in the range of about 5 to 10 μm in diameter. For example, amicrochannel plate blank having a diameter of 1 inch may have about 3 to5 million microchannels. The microchannels can have a length that is atleast 10 times the diameter of the microchannels. The bulk glass tubeand the core glass rod can have circular, square, rectangular, orhexagonal cross sections, resulting in microchannels that have circular,square, rectangular, or hexagonal cross sections.

Step 2: A layer of neutron sensitive material is deposited 204 on thesurface of the microchannel plate blank using atomic layer deposition.For example, the layer of neutron sensitive material can be the layer104 of FIG. 1A. The layer of neutron sensitive material can includeboron-10, lithium-6 or gadolinium, or a compound that includes boron-10,lithium-6 or gadolinium. The compound can include boron-10 oxide,boron-10 nitride, lithium-6 oxide, or gadolinium oxide. The layer ofneutron sensitive material can have a thickness in a range from 0.1 to 5microns, or from 0.5 to 5 microns. The layer of neutron sensitivematerial can include hafnium, samarium, erbium, neodymium, tantalum,lutetium, europium, dysposium, and/or thulium, or a compound thatincludes the above material(s).

Step 3: A layer of semiconducting material is deposited 206 on the layerof neutron sensitive material using atomic layer deposition. Forexample, the layer of semiconducting material can be the layer 106 ofFIG. 1A. The layer of semiconducting material can have a thickness in arange from 25 to 1000 nm, or from 50 to 1000 nm. The semiconductingmaterial can include, for example, a AlZn_(x)O_(y) alloy film.

Step 4: A layer of electron emissive material is deposited 208 on thelayer of semiconducting material using atomic layer deposition. Forexample, the layer of electron emissive material can be the layer 108 ofFIG. 1A. The layer of electron emissive material can have a thickness ina range from 3 to 20 nm, or from 3 to 12 nm. The electron emissivematerial can include aluminum oxide (Al₂O₃) or magnesium oxide (MgO).

Step 5: The faces of the microchannel plate are coated 210 with nichrometo serve as the electrodes for application of a voltage to generateelectric fields in the microchannels. This completes the processing ofthe microchannel plate, which is ready for testing and packaging.

FIG. 8 is a diagram of a process 220 for using a microchannel plate todetect neutrons based on the coincidence technique.

Step 1: A layer of neutron sensitive material formed on a wall of amicrochannel of a microchannel plate is used to capture 222 a neutronand generate at least one reactant particle from the capture of theneutron. The microchannel plate can include a glass plate having astructure that defines the microchannel, and the glass plate can includeglass having less than 5 mol % lead (Pb) and less than 1 mol % of theneutron sensitive material.

For example, the microchannel plate can be the microchannel plate 100 ofFIG. 1A. The layer of neutron sensitive material can be the layer 104.The layer of neutron sensitive material can include boron-10 orgadolinium, or a compound that includes boron-10 or gadolinium. Thecompound can include boron-10 oxide, boron-10 nitride, or gadoliniumoxide. The layer of neutron sensitive material can have a thickness in arange from 0.5 to 5 microns. The layer of neutron sensitive material canalso include other elements that release gamma rays upon neutroncapture.

Step 2: Secondary electrons are generated 224 based on an interactionbetween the reactant particle and a layer of electron emissive materialon the wall of the microchannel. For example, the layer of electronemissive material can be the layer 108 of FIG. 1A. The layer of electronemissive material can have a thickness in a range from 3 to 12 microns.The electron emissive material can include aluminum oxide (Al₂O₃) ormagnesium oxide (MgO).

Step 3: The secondary electrons are detected 226. For example, a signalpulse from the anode 160 (or the output electrode 152) can indicatedetection of secondary electrons.

Step 4: A gamma ray is detected 228. For example, the gamma ray detector174 can be used to detect a gamma ray and generate a signal indicatingthat a gamma ray has been detected.

Step 5: Determine 230 whether the signal indicating detection of a gammaray occurred within a specified time period after occurrence of thesignal indicating detection of secondary electrons.

Step 6: If the signal indicating detection of a gamma ray occurredwithin the specified time period after the occurrence of the signalindicating detection of secondary electrons, a signal indicatingdetection of a neutron is generated 232.

FIG. 9 is a diagram of a process 280 for using a microchannel plate todetect neutrons based on the induced pulse technique.

Step 1: A layer of neutron sensitive material formed on a wall of amicrochannel of a microchannel plate is used to capture 282 a neutronand generate at least one reactant particle from the capture of theneutron. The microchannel plate can include a glass plate having astructure that defines the microchannel, and the glass plate can includeglass having less than 5 mol % lead (Pb) and less than 1 mol % of theneutron sensitive material.

For example, the microchannel plate can be the microchannel plate 100 ofFIG. 1A. The layer of neutron sensitive material can be the layer 104.For detecting neutrons having energy of about 0.025 eV or less, thelayer of neutron sensitive material can include boron-10, lithium-6,gadolinium, or a compound that includes boron-10, lithium-6, orgadolinium. The compound can include boron-10 oxide, boron-10 nitride,lithium-6 oxide, gadolinium oxide. The layer of neutron sensitivematerial can have a thickness in a range from 0.1 to 5 microns, or from0.5 to 5 microns.

For detecting neutrons having energy of above 0.025 eV, the layer ofneutron sensitive material can include hafnium-177, samarium-149,erbium-167, neodymium, tantalum-180, lutetium, europium, dysposium,and/or thulium, or a compound that includes any of the above elements.The compound can include an oxide or a nitride of any of the aboveelements. The layer of neutron sensitive material can have a thicknessin a range from 0.1 to 5 microns, or from 0.5 to 5 microns.

Step 2: Secondary electrons are generated 284 based on an interactionbetween the reactant particle and a layer of electron emissive materialon the wall of the microchannel. For example, the layer of electronemissive material can be the layer 108 of FIG. 1A. The layer of electronemissive material can have a thickness in a range from 3 to 12 microns.The electron emissive material can include aluminum oxide (Al₂O₃) ormagnesium oxide (MgO).

Step 3: A first charge induced on an input electrode of the microchannelplate and a second charge induced on an output electrode of themicrochannel plate are detected (286). For example, a first chargeinduced on the input electrode 150 of the microchannel plate 100 and asecond charge induced on the output electrode 152 of the microchannelplate 100 can be detected.

Step 4: A ratio between the first charge and the second charge, and asum of the first and second charges, are calculated (288). A parametervalue is calculated based on dividing the ratio by the sum.

Step 5: The parameter value is compared with a predetermined range ofvalues, and if the parameter value is within the predetermined range ofvalues, a signal is generated (290) indicating that a neutron has beendetected.

It is to be understood that the foregoing description is intended toillustrate and not to limit the scope of the neutron detection devicesdescribed herein. For example, the microchannel plate can have one ormore neutron sensitive materials (e.g., boron-10, lithium-6, and/orgadolinium, and/or one or more of their compounds) in the bulkstructure, and have a layer of semiconducting material and a layer ofelectron emissive material disposed on walls of the microchannels, inwhich no additional layer of neutron sensitive material is used. Thelayer of electron emissive material can have a thickness in a rangefrom, e.g., 5 to 10 nm thick, and be made of a material that is aninsulator such as SiO₂, Al₂O₃, or MgO, which emits secondary electronsdirectly into the open microchannels. The semiconducting (or“resistive”) layer can be about 50 nm to 500 nm thick, is positionedbeneath the electron emissive layer, and carries an electron biascurrent. These free electrons resupply/recharge the positive holes leftbehind in the overlying electron emitting layer.

Gadolinium can be used to effectively detect thermal neutrons (around0.025 eV). Gadolinium also has fifteen resonances between 1 eV and 25eV, and can be used to detect neutrons having higher energy levels.

In some examples, the microchannel plate can have neutron sensitivematerial in both the bulk structure and in one of the layers disposed onwalls of the microchannels. The neutron sensitive material in the bulkstructure can be the same or different from the neutron sensitivematerial in the layer disposed on the walls of the microchannels. Insome examples, the bulk glass can be doped with one or more of boron-10,lithium-6, or gadolinium, or a compound that includes boron-10,lithium-6, or gadolinium. A layer of material that includes hathium,samarium, erbium, neodymium, tantalum, lutetium, europium, dysposium,thulium, gadolinium, or a compound that includes any of the above, canbe deposited on the walls of the microchannels. In some examples, thebulk glass can be doped with hafnium, samarium, erbium, neodymium,tantalum, lutetium, europium, dysposium, thulium, or gadolinium, or acompound that includes any of the above. A layer of material thatincludes boron-10, lithium-6, or gadolinium, or a compound that includesany of the above, can be deposited on the walls of the microchannels. Byusing two different neutron sensitive materials that are sensitive toneutrons having different energy levels, the microchannel plate can beused to detect neutrons having a wider range of energy levels.

Each of Hf, Sm, Er, Nd, Ta, Lu, Eu, Dy, Tm, and Gd can have multipleisotopes, each of which may absorb neutrons having different energylevels. Different isotopes of each element may be chosen depending onwhich specific neutron energy is to be converted into an internalconversion electron resulting in a detectable signal from themicrochannel plate. The elements, either in their naturally occurringforms or their isotopes, can be used singly or in combination in thebulk microchannel plate structure or in the coating layers on themicrochannel surfaces.

Each layer of neutron sensitive material can include two or more of B,Li, Hf, Sm, Er, Nd, Ta, Lu, Eu, Dy, Tm, or Gd, or compounds of theabove.

Two or more layers of neutron sensitive materials can be disposed on themicrochannel walls. For example, a first layer of neutron sensitivematerial that includes boron-10 or lithium-6 or a compound that includesone of the above can be formed on the microchannel walls, e.g., byatomic layer deposition. A second layer of neutron sensitive materialthat includes Hf, Sm, Er, Nd, Ta, Lu, Eu, Dy, Tm, or Gd, or a compoundthat includes one of the above, can be deposited on top of the firstlayer of neutron sensitive material, e.g., by atomic layer deposition.By having two or more layers of different types of neutron sensitivematerials, it is possible to detect neutrons having a greater range ofenergy levels. For example, the microchannel plate can be used to detectneutrons having energy levels ranging from less than 0.005 eV to as highas 1000 eV.

The microchannel plate can be made of glass, polymer, or plastic. Otherthan the process 200, a microchannel plate can also be fabricated usinga so-called “hollow draw” process. One approach in hollow draw is tobundle many parallel hollow glass fibers together. The hollow fiber isproduced by pulling larger glass tubing through an oven, and pulling thefiber vertically downwards as the larger tubing is fed into the oven ata slower rate. The hollow fiber is carefully produced to have a constantdiameter, taking in account the rates at which the tubing is fed intothe oven, the rate the fiber is pulled downwards, and the oventemperature. In this procedure, fiber size is critically maintained andcontrolled. The diameter reduction ratio from tube to drawn hollow fiberis determined by the square root of the input tube feed to output fibervelocity ratio, for conservation of volume and mass. The fiberthicknesses are carefully monitored during the draw process, often usinglaser micrometers. The fibers are then bundled together in a mold andfused. Microchannel plates are fabricated by fusing the bundles, andthen slicing each bundle into about 1 mm thick wafers. The bundle can bepolygonal in shape, e.g., square, circular, octagonal, and so forth. Thebundle or the individual wafer is cut into the desired shape and formatsize. To prevent debris in the form of broken glass or polishing mediafrom lodging within the hollow microchannels, the channels can first befilled with a wax or other soluble medium, and then later dissolved andremoved. The two faces of the wafers are polished, and the electricalproperties are developed using either (i) hydrogen reduction of leadoxide present in the base microchannel plate glass, or (ii) atomic layerdeposition of two different functional layers, an underlyingsemiconducting layer and a thin (about 10 nm) insulating electronemissive layer applied on top, with this combined coating forming themicrochannel inner wall activated surface.

The microchannel plate detector 172 can include several microchannelplates 100 stacked together. The dimensions of the microchannels 110 andother parameters can be different from those described above. Boron-10,lithium-6 and gadolinium may exist in other forms. The resistive layer106 can include materials other than those described above. For example,the resistive layer 106 can include a mixture of one or more conductiveand one or more insulating materials so that the resistive layer 106 canhave semiconducting properties. The conductive materials may includeZnO, SnO₂, In₂O₃, and the insulating materials may include Al₂O₃ andTiO₂.

In some examples, the low-Z bulk glass that is used to form the fibersfor fabricating the glass plate 102 can have zero or a low percentage ofneutron sensitive material (e.g., having less than 0.1 mol %). In someexamples, the low-Z bulk glass can include neutron sensitive material(e.g., having more than 0.1 mol %, or more than 10 mol % boron). In thelatter case, the coating of neutron sensitive material in the surface ofthe microchannel enhances the sensitivity of the neutron detector 100 toneutrons.

In some implementations, a microsphere plate can be used instead of amicrochannel plate. For example, the neutron detector 170 (FIG. 5) orneutron detector 250 (FIG. 6) can include a microsphere plate detectorinstead of the microchannel plate detector 172. A microsphere plate caninclude a glass plate formed of microscopic glass spheres. The glassspheres can have a diameter equal to or less than, for example, 100microns. The microscopic glass spheres can be coated with a layer ofneutron sensitive material (similar to layer 104), a layer ofsemi-conductive material (similar to layer 106), and a layer of electronemissive material (similar to layer 108). The neutron sensitivematerial, semi-conductive material, and electron emissive material canbe formed on the surfaces of the microspheres using, for example, atomiclayer deposition. The spheres are packed and bonded together, e.g., bycompression and sintering. Electrodes are attached to the top and bottomfaces of the microsphere plate to allow high bias voltages to be appliedto the microsphere plate. As incident particles collide against thesurfaces of the spheres to form secondary electrons, a cascade ofelectrons can be formed as the secondary electrons accelerate throughthe interstices defined by the spheres and collide against the surfacesof other spheres. The microspheres can include bodies having othershapes, such as oval-shaped bodies or irregularly-shaped bodies.

In some implementations, a microfiber plate can be used instead of amicrochannel plate. For example, the neutron detector 170 (FIG. 5) orneutron detector 250 (FIG. 6) can include a microfiber plate detectorinstead of the microchannel plate detector 172. A microfiber plate caninclude multiple layers of microfibers that are laminated together. Themicrofibers can have a diameter equal to or less than, for example, 100microns. Prior to lamination, the microfibers can be coated with a layerof neutron sensitive material (similar to layer 104), a layer ofsemi-conductive material (similar to layer 106), and a layer of electronemissive material (similar to layer 108). The neutron sensitivematerial, semi-conductive material, and electron emissive material canbe formed on the surfaces of the microfibers using, for example, atomiclayer deposition. In some examples, sheets of microfibers are stackedtogether, and the stack of microfibers are fused at a high temperaturein an oven, such that the contact points of the individual microfibersare slightly fused together. A layer of nichrome is coated on each ofthe top and bottom faces of the microfiber plate. The nichrome layersserve as electrodes that allow high bias voltages to be applied to themicrofiber plate. As incident particles collide against the surfaces ofthe microfibers to form secondary electrons, a cascade of electrons canbe formed as the secondary electrons accelerate through the intersticesdefined by the microfibers and collide against the surfaces of othermicrofibers. The cross sections of the microfibers can have variousshapes, such as circular, oval, square, or irregular shaped crosssections.

Elements of different embodiments described above may be combined toform embodiments not specifically described herein. Otherimplementations not specifically described herein are also within thescope of the following claims.

What is claimed is:
 1. An apparatus comprising: a microchannel platecomprising a structure that defines a plurality of microchannels; andlayers of materials disposed on walls of the microchannels, the layersincluding a layer of neutron sensitive material, a layer ofsemiconducting material, and a layer of electron emissive material, inwhich the layer of neutron sensitive material comprises at least one ofhafnium (Hf), samarium (Sm), erbium (Er), neodymium (Nd), tantalum (Ta),lutetium (Lu), europium (Eu), dysposium (Dy), or thulium (Tm).
 2. Theapparatus of claim 1 in which the layer of neutron sensitive materialcomprises at least 50 mol % of neutron sensitive material.
 3. Theapparatus of claim 1 in which the layer of neutron sensitive materialcomprises at least 30 mol % of neutron sensitive material.
 4. Theapparatus of claim 1 in which the layer of neutron sensitive materialcomprises at least one of boron-10, lithium-6, or gadolinium.
 5. Theapparatus of claim 1 in which the layer of neutron sensitive materialcomprises a compound that comprises at least one of boron-10, lithium-6,or gadolinium, and the compound comprises at least one of boron-10oxide, boron-10 nitride, lithium-6 oxide, or gadolinium oxide.
 6. Theapparatus of claim 1 in which the structure comprises at least one ofglass, polymer, or plastic.
 7. The apparatus of claim 1 in which thesemiconducting material comprises AlZn_(x)O_(y) alloy, x and y beingpositive integers.
 8. The apparatus of claim 1 in which the electronemissive material comprises at least one of aluminum oxide (Al₂O₃) ormagnesium oxide (MgO).
 9. The apparatus of claim 1, comprising a gammaray detector to detect gamma rays, and a coincidence unit to determinewhether a signal output from the gamma ray detector indicating detectionof a gamma ray occurs within a predetermined time period after a signaloutput from the microchannel plate indicating detection of at least oneof a neutron or a gamma ray.
 10. The apparatus of claim 1 in which themicrochannel plate comprises an input electrode, an output electrode,and a glass plate comprising the microchannels, and the apparatusfurther comprising a data processor to determine whether a neutron hasbeen detected based on first information derived from a first chargeinduced on the input electrode and/or second information derived from asecond charge induced on the output electrode.
 11. The apparatus ofclaim 1 in which the microchannel plate comprises an input electrode, anoutput electrode, and a glass plate comprising the microchannels, andthe apparatus further comprising a data processor to determine whether aneutron has been detected based on a comparison of a first signal on theinput electrode and a second signal on the output electrode.
 12. Theapparatus of claim 11 in which the data processor determines that aneutron has been detected when the first and second signals havedifferent polarities.
 13. The apparatus of claim 1 in which themicrochannel plate comprises an input electrode, an output electrode,and a glass plate comprising the microchannels, and the apparatusfurther comprising a data processor to determine whether a neutron hasbeen detected based on a comparison of a signal on the output electrodewith a baseline value.
 14. The apparatus of claim 1 in which themicrochannel plate is capable of detecting neutrons having energy levelsin a range from 0.005 eV to 1000 eV.
 15. The apparatus of claim 1 inwhich the microchannel plate is capable of detecting neutrons havingenergy levels in a range from 0.025 eV to 10 eV.
 16. The apparatus ofclaim 1 in which the microchannel plate is capable of detecting neutronshaving energy levels in a range from 1 eV to 300 eV.
 17. The apparatusof claim 1 in which the microchannel plate is capable of detectingneutrons having energy levels in a range from 300 eV to 1000 eV.
 18. Amethod of fabricating a microchannel plate, the method comprising:fabricating a structure that defines a plurality of microchannels; anddepositing a layer of neutron sensitive material, a layer ofsemiconducting material, and a layer of electron emissive material onwalls of the microchannels, in which the layer of neutron sensitivematerial comprises at least one of hafnium (Hf), samarium (Sm), erbium(Er), neodymium (Nd), tantalum (Ta), lutetium (Lu), europium (Eu),dysposium (Dy), or thulium (Tm).
 19. The method of claim 18 in whichdepositing a layer of neutron sensitive material comprises using atomiclayer deposition to deposit a layer of neutron sensitive material. 20.The method of claim 19 in which using atomic layer deposition to deposita layer of neutron sensitive material comprises using atomic layerdeposition to deposit at least two of boron-10, lithium-6, gadolinium,hafnium, samarium, erbium, neodymium, tantalum, lutetium, europium,dysposium, or thulium.
 21. The method of claim 18 in which depositing alayer of semiconducting material comprises using atomic layer depositionto deposit a layer of semiconducting material.
 22. The method of claim18 in which depositing a layer of electron emissive material comprisesusing atomic layer deposition to deposit a layer of electron emissivematerial.
 23. The method of claim 18 in which fabricating a structurethat defines a plurality of microchannels comprises fabricating astructure using a plurality of fibers each including a soluble core anda layer of cladding surrounding the soluble core, and removing thesoluble core to form microchannels.
 24. The method of claim 18 in whichfabricating a structure that defines a plurality of microchannelscomprises drawing of hollow tubes and bundling the drawn hollow tubes.25. The method of claim 18 in which depositing a layer of neutronsensitive material, a layer of semiconducting material, and a layer ofelectron emissive material on walls of the microchannels comprisesdepositing a layer of neutron sensitive material, followed by depositinga layer of semiconducting material, and followed by depositing a layerof electron emissive material on walls of the microchannels.
 26. Amethod of detecting neutrons, the method comprising: using a layer ofneutron sensitive material formed on a wall of a microchannel of amicrochannel plate to capture a neutron and generate at least onereactant particle, the microchannel plate comprising a glass platehaving a structure that defines the microchannel, the glass platecomprising glass having less than 25 mol % lead (Pb) and less than 20mol % of the neutron sensitive material, in which the neutron sensitivematerial comprises at least one of hafnium (Hf), samarium (Sm), erbium(Er), neodymium (Nd), tantalum (Ta), lutetium (Lu), europium (Eu),dysposium (Dy), or thulium (Tm); detecting secondary electrons that aregenerated based on an interaction between the reactant particle and alayer of electron emissive material on the wall of the microchannel; andgenerating a first signal indicating detection of a neutron.
 27. Themethod of claim 26, further comprising generating a second signalindicating detection of the secondary electrons, generating a thirdsignal indicating detection of a gamma ray, determining whether thethird signal occurred within a specified time period after occurrence ofthe second signal, and generating the first signal only if the thirdsignal occurred within the specified time period after occurrence of thesecond signal.
 28. The method of claim 26 in which the glass platecomprises glass having less than 1 mol % lead (Pb).
 29. The method ofclaim 26 in which the glass plate comprises glass having less than 0.1mol % of the neutron sensitive material.
 30. An apparatus comprising: aneutron detector configured to detect neutrons having energy higher than0.025 eV, the neutron detector including: a microchannel plate having astructure that defines a plurality of microchannels, in which thestructure includes a neutron sensitive material, the neutron sensitivematerial including at least one of hafnium (Hf), samarium (Sm), erbium(Er), neodymium (Nd), tantalum (Ta), lutetium (Lu), europium (Eu),dysposium (Dy), or thulium (Tm).
 31. The apparatus of claim 30 in whichthe neutron sensitive material comprises a compound including at leastone of hafnium (Hf), samarium (Sm), erbium (Er), neodymium (Nd),tantalum (Ta), lutetium (Lu), europium (Eu), dysposium (Dy), or thulium(Tm).
 32. The apparatus of claim 30 in which the neutron detector isalso configured to detect neutrons having energy equal to or lower than0.025 eV, and the neutron sensitive material comprises at least at oneboron-10, lithium-6, or gadolinium.
 33. The apparatus of claim 30 inwhich the structure comprises glass.
 34. The apparatus of claim 30 inwhich a layer of neutron sensitive material and a layer ofsemiconducting material are disposed on walls of the microchannels. 35.The apparatus of claim 34 in which the semiconducting material comprisesAlZn_(x)O_(y) alloy, x and y being positive integers.
 36. The apparatusof claim 34 in which the electron emissive material comprises at leastone of aluminum oxide (Al₂O₃) or magnesium oxide (MgO).
 37. Theapparatus of claim 30, comprising a gamma ray detector to detect gammarays, and a coincidence unit to determine whether a signal output fromthe gamma ray detector indicating detection of a gamma ray occurs withina predetermined time period after a signal output from the microchannelplate indicating detection of at least one of a neutron or a gamma ray.38. The apparatus of claim 30 in which the microchannel plate comprisesan input electrode, an output electrode, and a glass plate comprisingthe microchannels, and the apparatus further comprising a data processorto determine whether a neutron has been detected based on firstinformation derived from a first charge induced on the input electrodeand second information derived from a second charge induced on theoutput electrode.
 39. The apparatus of claim 30 in which themicrochannel plate comprises an input electrode, an output electrode,and a glass plate comprising the microchannels, and the apparatusfurther comprising a data processor to determine whether a neutron hasbeen detected based on a comparison of a first signal on the inputelectrode and a second signal on the output electrode.
 40. The apparatusof claim 39 in which the data processor determines that a neutron hasbeen detected when the first and second signals have differentpolarities.
 41. The apparatus of claim 30 in which the microchannelplate comprises an input electrode, an output electrode, and a glassplate comprising the microchannels, and the apparatus further comprisinga data processor to determine whether a neutron has been detected basedon a comparison of a signal on the output electrode with a baselinevalue.
 42. A method of fabricating a neutron detector, the methodcomprising: fabricating a microchannel plate, comprising: fabricating astructure that defines a plurality of microchannels, including dopingglass with a neutron sensitive material comprising at least one ofhafnium (Hf), samarium (Sm), erbium (Er), neodymium (Nd), tantalum (Ta),lutetium (Lu), europium (Eu), dysposium (Dy), or thulium (Tm), andforming the structure using the glass doped with neutron sensitivematerial; and attaching an input electrode and an output electrode tothe microchannel plate.
 43. The method of claim 42 in which fabricatingthe microchannel plate comprises using atomic layer deposition todeposit a layer of semiconducting material.
 44. The method of claim 42in which fabricating the microchannel plate comprises using atomic layerdeposition to deposit a layer of electron emissive material.
 45. Themethod of claim 42 in which fabricating a structure that defines aplurality of microchannels comprises fabricating a structure using aplurality of fibers each including a soluble core and a layer ofcladding surrounding the soluble core, and removing the soluble core toform microchannels.
 46. The method of claim 42 in which fabricating themicrochannel plate comprises depositing a layer of semiconductingmaterial, followed by depositing a layer of electron emissive materialon walls of the microchannels.
 47. A method of detecting neutrons, themethod comprising: using a microchannel plate to capture a neutron andgenerate at least one reactant particle, the microchannel platecomprising a glass plate having a structure that defines themicrochannel, the glass plate comprising glass having less than 25 mol %lead (Pb), the glass comprising a neutron sensitive material comprisingat least one of hafnium (Hf), samarium (Sm), erbium (Er), neodymium(Nd), tantalum (Ta), lutetium (Lu), europium (Eu), dysposium (Dy), orthulium (Tm); detecting secondary electrons that are generated based onan interaction between the reactant particle and a layer of electronemissive material on the wall of the microchannel; and generating afirst signal indicating detection of a neutron.
 48. The method of claim47, further comprising generating a second signal indicating detectionof the secondary electrons, generating a third signal indicatingdetection of a gamma ray, determining whether the third signal occurredwithin a specified time period after occurrence of the second signal,and generating the first signal only if the third signal occurred withinthe specified time period after occurrence of the second signal.
 49. Themethod of claim 47 in which the glass plate comprises glass having lessthan 1 mol % lead (Pb).
 50. An apparatus comprising: a microchannelplate comprising a structure that defines a plurality of microchannels;and layers of materials disposed on walls of the microchannels, thelayers including a first layer of a first neutron sensitive material, asecond layer of a second neutron sensitive material, a layer ofsemiconducting material, and a layer of electron emissive material, inwhich first neutron sensitive material is different from the secondneutron sensitive material.
 51. The apparatus of claim 50 in which thefirst neutron sensitive material comprises at least one of boron,lithium, or gadolinium, and the second neutron sensitive materialcomprises at least one of hafnium, samarium, erbium, neodymium,tantalum, lutetium, europium, dysposium, thulium, or gadolinium.
 52. Theapparatus of claim 50 in which the first neutron sensitive material issensitive to neutrons having an energy level equal to or less than 0.025eV, and the second neutron sensitive material is sensitive to neutronshaving an energy level higher than 0.025 eV.